1. Field of the Invention
The present invention relates to driving an inkjet recorder equipped with a recording head that records characters or graphics by pressurizing and squirting ink through use of a piezoelectric element.
2. Related Art
Ink is used in inkjet print heads. For the proper operation of inkjet print heads, it is important that the ink have consistent physical properties. It is not always possible, however, for the physical properties of the ink to be identical because of various external factors. One such external factor affecting the physical properties of inkjet ink is ambient temperature. In particular, the ambient temperature of the print head affects the viscosity and the surface tension of inkjet ink. That is, the lower the ambient temperature, the more viscous the ink becomes. The higher the ambient temperature, the less viscous the ink becomes.
To put it another way, inkjet ink has some physical properties that are temperature dependent. The temperature dependent physical properties of inkjet ink result in a variation in the squirting characteristics of an ink droplet (such as squirting velocity or droplet weight), thereby adversely affecting print quality.
One exemplary method of driving an inkjet print head is disclosed in the International Patent Application (IPA) WO 95/16568, for which the international publication has been effected. According to this IPA, a piezoelectric element at an intermediate drive voltage is discharged to the minimum drive voltage, thereby to draw ink into a pressure chamber by suction.
In other words, a pressure generating chamber is expanded, which pulls ink from an ink reservoir into the pressure generating chamber. Immediately after the sucking/expanding operation, the piezoelectric element is charged to the maximum voltage, thereby to squirt ink. In other words, the pressure generating chamber is contracted, forcing an ink droplet out. Immediately after the squirting of ink, the piezoelectric element is discharged back to the intermediate drive voltage. That is, in the basis drive method of the above-identified IPA, the pressure generating chamber first is expanded and draws ink into the pressure generating chamber and then the pressure generating chamber is contracted, pushing an ink droplet out from the chamber
The basic drive method according to the above-identified IPA provides a good example of the negative effects that temperature can have on printing quality. This basic drive method does not take into account temperature. If the drive method of the above-identified IPA is used in the entire range of likely ambient temperatures, and particularly for drive frequencies at or over 20 kHz, problems are encountered. Such problems will shortly be noted, but first it is important to note that the drive frequency of an inkjet print head may generally be understood to mean the number of times ink droplets are squirted per unit of time. It is also important to note that the jetting or squirting of an ink droplet is an operation in which residual vibrations are encountered, and that at frequencies of 20 kHz or more, the influence of the residual vibration of an ink meniscus manifests itself.
Now, the problems encountered in the drive method according to the above-identified IPA will be pointed out. As the ambient temperature decreases, as has already been mentioned, the viscosity of the ink increases. In other words, the ink becomes thicker. The amount of ink drawn into the pressure generating chamber during the expansion of the chamber thus is decreased in relation to the increase in ink viscosity. In addition, the higher ink viscosity causes the meniscus of the ink to return to a discharge orifice at a slower rate after the squirting operation. This means that the next ink droplet gets squirted when the meniscus has not yet returned to the ideal, higher ambient temperature position. To put it another way, because the ink is more viscous at the lower temperature, the ink meniscus might not return to the proper position in time for the jetting of the next ink droplet; the meniscus might be too deep within the pressure generating chamber. This improper positioning of the meniscus causes the amount of jetted ink to be reduced correspondingly and, moreover, the jetted droplet might have a linear shape instead of the desired granular shape. Overall, the foregoing factors combine to produce a significant decrease in the total amount of ink jetted. This decrease in the total amount of ink jetted results in the inevitable degradation of picture quality.
One approach to avoid the adverse effects of the temperature dependent physical properties of inkjet ink is to ensure the ink is always near a particular temperature. Laid-open JPA Hei. 5-220947 exemplifies this approach. In particular, this JPA provides that, when the ambient temperature is low, the ink is heated to a temperature close to room temperature before being squirted. This technique is not entirely advantageous. Although the temperature dependency of the physical properties is ameliorated, it is necessary to provide an ink heater. The addition of an ink heater adds to the manufacturing cost of the print head.